The present invention relates to an image forming apparatus wherein a visualized image is formed in accordance with signals representing an image to be recorded.
A laser beam printer utilizing electrophotography, is known as an example of apparatus wherein a visualized image is formed from image signals provided by a computer, a word processing system or an image sensor which reads an original and produces the image signals.
FIG. 1 illustrates an example of a laser beam printer, which comprises an image bearing member in the form of a photosensitive drum 101 including a base plate 103 and an electrophotographic photosensitive layer thereon, made of for example a phthalocyanine organic semiconductor. Around the photosensitive drum 101, there are provided a primary charger 104 and developing means 106 including a doctor blade 107, a hopper 107', a magnet roller 108 and a sleeve 109. In the hopper 107', a toner developer is contained, which is for example one component magnetic developer. Around the photosensitive drum 101, there are further provided a pre-exposure lamp 110, a guide for image transfer paper 111, a transfer charger 112 and a cleaner including a rubber blade 114 a magnet roller 115, a screw 116 and a cleaner housing 117.
The laser beam printer includes a semiconductor laser 2 which emits a laser beam 105 when it is modulated and driven by a modulator 1 in response to an image signal fv which corresponds to the image to be recorded. That is, the laser beam 105 is modulated in accordance with the image signal to be recorded. The laser beam 105 is scanningly deflected by a rotatable polygonal mirror 118 to scan the photosensitive drum which is rotating. The direction of scanning is perpendicular to the rotational movement of the photosensitive drum 101. The laser beam 105 is imaged on the photosensitive drum 101 as a spot by a lens 119.
The photosensitive drum 101 is uniformly charged by the primary charger 104 to the negative polarity, for example, and then exposed to the laser beam 105 modulated by the image signal to be recorded, whereby an electrostatic latent image is formed on the drum 101. The electrostatic latent image is visualized with the toner T by the developing means 106. The visualized image is then transferred by the transfer charger 112 onto the transfer paper introduced thereto by way of the transfer paper guide 111. The transferred toner image is fixed on the transfer paper by an unshown image fixing device, whereafter the transfer paper is discharged out of the printer.
The residual toner which has not been transferred onto the transfer paper and remains on the photosensitive drum 101, is removed from the surface of the drum 101 by the rubber blade 114, and then attracted to the magnet roller 115. The attracted toner particles are collected by a screw 116 into a toner collecting box (not shown) which is formed as a part of the cleaner housing 117. After the cleaning, the photosensitive drum 101 is electrically discharged by the lamp 110 so as to be prepared for the next step initiating with the primary charging. Thus, the photosensitive drum is repeatedly used for the image formation.
For a better understanding, FIG. 2 shows the change in the surface potential of the photosensitive drum 101 with the process of the above steps. In this Figure, the surface potential at the area not exposed to the laser beam 105 is indicated by the reference Vd, whereas the surface potential at the area exposed thereto is indicated by Ve. The difference between the potentials Vd and Ve is for example 550 V.
Generally, the laser beam printer uses a so-called image scanning type system wherein the point or part of the photosensitive drum 101 to be visualized, that is, the point or part which is to receive the toner, is exposed to the laser beam, while the part of the photosensitive drum where the toner is not substantially deposited, is not exposed to the laser beam. It is preferable in the present invention to utilize this system. This is because there is no trace of scan appearing in the background of the image and because the reproducibility of the visualized image is better, as compared with a so-called background scanning type system wherein the laser beam 105 is projected to the part of the photosensitive drum 101 which corresponds to the background of the image, while the laser beam 105 is not projected to the point or part corresponding to the point or part to be visualized. In order to deposit the toner to such a part of the photosensitive drum 101 that has been exposed to the laser beam 105, that is, the part having the surface potential Ve, the toner T is of the polarity the same as the charging polarity of the primary charger 104, as shown in FIG. 3. The bias voltage to be applied to the developing means has preferably a DC component voltage V which is between the potentials Vd and Ve. However, the present invention is not limited to the image scanning type system, but is applicable to the background scanning type system, wherein the toner T is of the polarity which is the opposite to the charging polarity of the primary charger 104.
In any case, the sleeve 109 of the developing means 106 receives the toner T from the hopper 107' and carries it thereon. During the carrying, the toner T is regulated into a toner layer of a uniform thickness on the sleeve 109 by the doctor blade 107. With the rotation of the sleeve 109, the regulated toner layer is conveyed to the portion where the sleeve 109 faces the photosensitive drum 101. To the sleeve 109, the developing bias voltage is applied by a power source 122 consisting of a variable DC source 120 and a variable AC source 121. Thus, an electric field is formed between the bias potential and the potential of the electrostatic latent image potential, which electric field is effective to attract the toner from the sleeve 109 to the latent image to be developed. The toner T is triboelectrically charged by the friction among the toner particles and the friction of the toner particles with the sleeve 109, the doctor blade 107 or the like.
With continuation and integration of the printer operation, the toner T is consumed to such an extent that the amount of the toner T within the hopper 107' becomes insufficient, resulting in voids in the image. To avoid this, as shown in FIG. 4 a sensor 21 employing a piezoelectric element may be provided within the developing means 106 to detect the low level of the toner in response to the change in the sensor output. More particularly, the output voltage of the piezoelectric element changes with the weight of the toner T, and the output voltage is compared by a comparator with a predetermined reference voltage. When the comparison indicates the low level of the toner, an indication lamp 23 is lit, thus warning the operator.
However, the warning system involves a drawback since if the toner T is non-uniformly distributed to overlie the piezoelectric element 21, the lamp 23 is not turned on even if the toner level has become low, because of the still heavy weight of the toner on the piezoelectric element 21. This, of course, may result in the voids in the image. On the contrary, if the toner T is concentrated apart from the piezoelectric element 21, the weight to the piezoelectric element 21 is small despite the sufficient amount of the toner T existing in the hopper and the lamp 23 will indicate a low level of the toner T. If the toner is supplied in response to such an erroneous warning, the developing means 106 contains too much toner T, with the result that the toner T is caked therewithin, so that the image density is lowered at such a portion, or the toner fuses and sticks to the sleeve 109, whereupon a non-uniform image is formed.
Japanese Laid-Open Patent Application No. 224363/1983 proposes that the number of dots constituting the electrostatic latent image be counted, and the toner consumption is predicted on the basis of the number counted.